“While We’re Young” opens with an exchange from Ibsen’s “The Master Builder” about what happens when the young come knocking at the door. But it might just as well begin with the wry Amish aphorism, “We grow too soon old and too late smart.”

Writer/director Noah Baumbach’s work often centers on the perils, agonies, and humiliations, and fears of growing up — including the attempts to avoid it. From the 20-somethings of “Kicking and Screaming” and “Frances Ha” to the immature parents with a teenager struggling through adolescence in his most autobiographical film, “The Squid and the Whale,” he shows us characters who try –unsuccessfully — to hold on to the optimism and narcissism of youth while having access to the powers and privileges of adulthood. Wouldn’t that be nice?

One of the toughest losses of adulthood is the sense of limitless possibilities. That is the moment at which we meet Josh (Ben Stiller) and Cornelia (Naomi Watts). Their friends all have babies. But after miscarriages and failed fertility treatments, they are giving up on having children and trying to convince themselves that they are happy to have nothing to keep them from spontaneous adventures — even if they never take them. Maybe planning a month ahead of time can still be spontaneous, but they don’t do that, either.

Josh is a documentary filmmaker who has been working on the same amorphous film for eight years. The version he is currently editing is so long it seems like it would take eight years to watch it. One problem is that he is not able to explain what it is about. Or, rather, it is about everything, including the very reality of being able to make a movie about whatever it is about. Also about America. So he is pretty much stalled in his personal and professional life.

And then he meets the adorably artisanal newlyweds Jamie (Adam Driver) and Darby (Amanda Seyfried), who have all the dewiness and boundless optimism Josh would love to feel. And, they have something even more important. They are so young that they think he is cool. They attend a class he is teaching and tell him they admire one of his documentary films, which they found on eBay. Josh and Cornelia are enraptured by the younger couple, who remind them of what they once were, while their old friends remind them of what they cannot have. In one sequence, Cornelia disastrously agrees to accompany her closest friend to a music session for infants, then runs out to join Darby at a hip hop dance class. She is hilariously out of place at both, but loves the feeling of being included with the hip hoppers.

Meanwhile, Josh starts wearing a hat just like Jamie’s and riding bicycles with him through the Brooklyn streets. But Josh’s best friend (played by Beastie Boy Adam Horovitz) tells Josh he’s “an old man with a hat” and Josh gets out of breath on the bike. Jamie calls Josh “Joshie,” affectionate but also infantilizing.

Both young and old will find a lot to laugh at in this film, which has Baumbach’s frothiest dialog and shrewdest characterizations.  “Arthritis arthritis?” Josh asks his doctor in dismay when he gets a diagnosis, hoping that perhaps he has some sort of specialized temporary form of arthritis that young people get.  “I usually just say ‘arthritis,'” his doctor dryly replies.  Josh and Cornelia are mesmerized by the preciously retro decor of Jamie’s and Darby’s apartment, with rows of LPs.  “It’s like they have everything we threw out but with them it looks good,” says Cornelia.

Then Jamie’s own documentary starts coming together, and Josh turns from mentor to stepping stone.  The documentarian’s obsession with truth-telling takes a twist. And the issue of fathers and sons takes another, as we learn that Cornelia’s father was a one-time mentor of Josh’s and is a possible mentor for Jamie.  

The final scene brings us full circle.  Baumbach’s past films have been perceptive and wryly funny, sometimes sympathetic, but here we get to see some tenderness for his characters.  As former poet laureate Billy Collins says when people use the word some critics are using about this film: “accessible,” this is not “accessible.”  It is welcoming.

Parents should know that this movie has very strong language and some drinking and drug use.

Family discussion: Who was right about Jamie’s movie? Ask the older people in your family what they like best about not being young anymore.

If you like this, try: “Metropolitan” and “Kicking and Screaming”